Arc welding and multipass friction stir welding ( Multi-FSW) were applied as the repair welding technique for tunnel defects of FSW joints. Microstructures and mechanical properties of repaired joints were analyzed and the influences of the repair welding process on joint properties were evaluated. The experimental results indicate that mechanical properties of Multi-FSW joints will not decrease obviously. Compared with FSW joints, repaired joints made using TIG + FSW have similar microstructures and show no microstructural deterioration. The repair weld鄄 ing method is proved to be effective and feasible for the repair of the longitude welding tunnel defect of a rocket fuel tank.